1. Field of the Invention
The present invention relates to an ultrathin walled rolling bearing for use in industrial robots, machine tools, medical equipment, and the like.
FIG. 5 illustrates an example of a CT scanner, a kind of medical equipment. As shown in the figure, in this CT scanner, an X-ray beam generated by an X-ray tube unit 20 is radiated toward a subject 23 via a wedge filter 21 for making the X-ray intensity distribution uniform, and a slit 22 for controlling the intensity distribution. The x-ray beam having passed through the subject 23 is received by a detector 24, is thereby converted into an electric signal, and is fed to a computer (not shown). The constituent components including the X-ray tube unit 20, the wedge filter 21, the slit 22, and the detector 24 are attached via a bearing 25 to a rotary platform 27 of substantially cylindrical shape rotatably supported by a stationary platform 26. As the rotary platform 27 is driven to rotate, these components are rotated about the subject 23. In the CT scanner, by driving the X-ray tube unit 20 and the detector 24 opposed to each other to rotate about the subject 23, projection data of the slice planes to be examined in multiple view directions around the subject 23 can be acquired. Then, by execution of an image reconstruction program configured beforehand based on the acquired projection data, a tomographic slice image is obtained.
In this CT scanner, the inner peripheral surface of the stationary platform 26 has a diameter large enough to permit the insertion of the subject 23 (a diameter of approximately 1 meter). Therefore, as the bearing portion 25 provided between the stationary platform 26 and the rotary platform 27, a bearing having a significantly small cross section relative to its diameter, i.e., a so-called ultrathin walled rolling bearing is used. As shown in FIG. 6, the ultrathin walled rolling bearing 25 is composed of an outer member 1xe2x80x2 having in its inner periphery a raceway surface 1bxe2x80x2, an inner member 2xe2x80x2 having in its outer periphery a raceway surface 2bxe2x80x2, a plurality of rolling elements 3xe2x80x2 (balls are used in the illustrated example) interposed between the raceway surfaces 2bxe2x80x2 and 1bxe2x80x2 of the inner and outer members 2xe2x80x2 and 1xe2x80x2, and a cage 4xe2x80x2 for circumferentially equally spacing the rolling elements 3xe2x80x2.
Among ultrathin walled rolling bearings, such a large-diameter bearing as has a pitch circle diameter (PCD) greater than 500 mm has its cage 4xe2x80x2 formed in a ring-like shape by joining together, for example, brass-based metal plates (segments) each having a circular section by welding. Each segment has a pocket for accommodating the rolling element 3xe2x80x2. This allows the rolling elements 3xe2x80x2 to be held circumferentially equidistantly. Between the pocket of the cage 4xe2x80x2 and the surface of the rolling element 3xe2x80x2 is formed a cage pocket clearance so as to provide flexibility in the mutual movement between them.
In a small-sized bearing, neither the inner peripheral surface nor the outer peripheral surface of the cage makes contact with the inner or outer member. That is, the cage is guided in rotation by the rolling element alone. On the other hand, in a large-diameter ultrathin walled rolling bearing as described above, the cage 4xe2x80x2 is guided in rotation by bringing the inner peripheral surface of the cage 4xe2x80x2 into contact with the outer peripheral surface of the inner member 2xe2x80x2, or bringing the outer peripheral surface of the cage 4xe2x80x2 into contact with the inner peripheral surface of the outer member 1xe2x80x2. In this case, the core deviation between the center of rotation of the bearing and that of the cage becomes unduly great. This increases the whirling movement of the cage, which may result in noise.
Moreover, depending upon the state of the inner or outer member and the cage in contact, an undesirable increase in the noise level may possibly occur.
An object of the present invention is to reduce the noise level of an ultrathin walled rolling bearing and improve the sound characteristics thereof.
To achieve the above object, according to the present invention, an ultrathin walled rolling bearing is provided with an outer member having in its inner periphery a raceway surface, an inner member having in its outer periphery a raceway surface, a plurality of rolling elements interposed between the raceway surfaces of the outer and inner members, and a cage for circumferentially equi-spacing the rolling elements. In this construction, the ratio of the diameter of the rolling element to the pitch circle diameter of the bearing is equal to or less than 0.03, and a ratio L/M falls within a range from 0.8 to 0.95 in which L is a radial thickness of the cage, and M is a radius difference between an inner peripheral surface of the outer member and an outer peripheral surface of the inner member.
Here, the range of the L/M value is wider than in conventional constructions. Accordingly, as compared with the conventional constructions, the radial movement of the cage 4 is restricted properly. This allows the center of rotation of the cage to come close to that of the bearing, and thereby the amount of radial whirling movement of the cage 4 is reduced. Consequently, occurrence of noise due to the whirling movement of the cage can be successfully prevented.
It is preferable that the ratio L/DB of the radial thickness L of the cage to the diameter DB of the rolling element be set to a range of 0.4 to 0.5.
Either or both of the inner and outer peripheral surfaces of the cage may have a lubricant storage portion formed thereon. The lubricant collected on this storage portion allows a sufficient supply of oil to the contact-making portions between the cage and the inner or outer member. Consequently, occurrence of noise due to the friction between the cage and the component making contact therewith can be successfully prevented.
The cage may be made of resin and is thus lighter in weight as compared to a metal-made construction. This makes further reduction in the noise level possible.
Of the outer and inner members, one may be fixed to a rotary platform of a CT scanner rotating about a subject, and the other may be fixed to a stationary platform of the CT scanner. This allows the rotary platform of the CT scanner to be rotatably supported with respect to the stationary platform.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.